US2321117A - Purification of naphthalene - Google Patents

Description

June 8, 1943.

Filed March 15, 1942.

MELTED IMPURI NAPHTHAL NE Ho-|- SOLUTION COOL TO.

cRYsTALLIzE /6 {i} (c RYsTA I s) WASHING C RYSTALS C RYSTALS WITH ADHERING soLvENT (WASHI Nes) 2 Sheetsf-Sheet 1 RECYCLED A LIQUOR PURIFIED NAPHTHALENE fraz/enfer f ofeal/ZCZZ? M'Zcoc/ l atented .lune 8, i943 assiali orne pany. Ciev Ghia, a corporation oi o. r

.application May is, i942, se No. aimez 9 ci. (Cl. 26.- 674) The present invention relates to the purincation of crude naphthaiene to provide a naphthavlene of higher melting point suitable for use in numerous chemical processes.

In many uses oi naphthalene a commercial crude product is unsatisfactory or undesirable, and it is necessary to rene it further, as to a point where it exhibits a solidification point (S. P.) of 79.8 C., has a wash test of a to 5 (discussed hereinafter) and an amber color when molten. Certain available crude naphthalenes having for example a solidication point of 78 C. or lower, do not meet these requirements. These and other crudes may be rei-ined by the present invention. Heretoore, renement has been effected commercially by Washing procedures, variously employing alkali solutions, acid solutions, and water, with or without a distillation step. One method is disclosed in U. S. patent to Todd and Owens No. 1,996,262. Well known theoretical or laboratory procedures involve recrystallization from numerous solvents. Naphthalene has a solubility in many varieties of solvent substances such-that it may be puried by a recrystallization operation.

However, such processes are expensive in that there is the necessity to redistill solvent for reuse of it in the process. The selection of a solvent for a commercial process depends in part upon the yield of purified material in proportion to the amount of solvent used, and also in proportion to the amount of solvent to be redistilled `ior continued eicient practice. Where the solvent material is a mixed one the question of distillation involves the problem of composition of the solvent mixture so that the process operates in the same way from time to time. Also where the solvent is a mixed solvent, it must be one which maintains a single liquid phase throughout therange of' temperatures encountered in the process oi' purifying the naphthalene.

Another practical consideration is the selection of a, solvent which has a relatively low boiling point, that is one such that the heating of it before cooling will not involve too expensive heating equipment or costs for heat. Likewise, one should be selected such that the cooling operation is not expensive.

The purpose of the present invention is to recrystallize naphthalene from a .solvent material meeting all of these requirements, and in addition-permitting a portion of the mother liquor to be recycled in the process when admixed with an maintaining a fixed ent hereinafter.

or redistilled -solvent material.

A.V particular object of the invention is to use as a solvent material, azeotropic mixtures of water and alcohols of the-group consisting oi.' ethyl, n-propyl. isopropyl, secondary-butyl and tertiary-butyl alcohols.

In carrying out the invention numerousv mechanical advantages First, however, the general procedure will be described without reference to .unnecessary mechanical detaill and without reerence to any particular one of the solvents. The

invention is explained with reference to the accompanying drawings in which: f

Fig. 1 is a diagrammatic arrangement of the steps ofthe process generally.

- that the resulting mixture is with melted naphthalene Fig. 2 is a diagrammatic arrangement of apparatus for 'conducting the process in a continuous It is to be understood that the invention and the apparatus are not in any way limited to or by the drawings, short of its scope as expressed in the appended claims.

, In Fig. 1, the numeral it represents naphthalene to be puriiied. `Preferably it is in a molten condition, because mixing the melted materialv with liquid solvent is easier than dissolving the crystals in hot solvent. 'Ihe temperature of the melted naphthalene can besuch in comparison with the temperature of the solvent to be used,

a. homogeneous solution not too much higher in temperature than the point where crystallization begins on cooling.

Numeral Il represents a. distilled solvent material. Numeral I2 represents a supply of the ,same solvent material containing impurities of the naphthalene, and ,it imay be considered as being a portion of the mother liquor derived from the process being described. Liquor i2 and a portion of `:fresh solution I3 to be cooled to throw out naphthalene.

The numeral M represents a cooling of the solution to provide a slush i5 comprising purer crystals of naphthalene, andsolvent liquor containing impurities derved from the naphthalene l0. Suitable separationy of the slush I5 into crystals it and liquor I7 is eiected accompanied ory followed by washing the `crystals at i8 to remove' adhering solvent liquor -with 'a' small amount of fresh solvent solvent liquor is represented by line 2 6 and the washed crystals with result as will become apparsupply of fresh or reliquor Ii are combined I0, to produce a heated il, the Washings beingl added at i9 to the said solvent liquor Il in dilu tion thereof. The diluted distillate which is too high sea level may employ an 2 adhering fresh solvent Il are represented by numeral 2l. The crystals 2| are melted in still 22, which discharges as a melt, purii'led naphthalene 23, and solvent vapor, which in condensed form on line 24 of the drawing is returned to supply l I.

The diluted solvent liquor 20 is divided into two portions 25 and 26. The portion 25 provides supply l2. The portion 26 is distilled in still 21 to recover and condense the solvent. `The recovered solvent 28 is added to the supply I I. The residue 29 from still 21 is the impurities removed from the crude naphthalene l in the puriilcation process.

From'the'foregoing it is seen that the melted naphthalene is dissolved in a mixture comprising recycled liquor I2, the original supply of solvent II, the recoveredsolvent 24, and the recovered solvent 28. It is important that these constituents be always of the same constitution. With a single solvent ingredient, as differentiated from a solvent mixture, this is readily effected. But with a solvent mixture it is not readily effected, except in the special circumstances hereinafter described.

Investigations have shown that the addition of water to pure single 'solvents which are miscible with water decreases the solvent power for naphthalene. This is desirable for high yield on cooling, so that too much naphthalene-is not recycled with the recycled liquor I2. Consequently, the present invention employs alcohols which are miscible ture which remains as a single phase in change of temperature from the mass I3 to the mass I5. Various mixtures of water and an alcohol will satisfy these conditions, but some of them will yieldin'stills 22 and 21 a distillate which is too low in alcohol content, and others will yield a in alcohol content, compared to the mixture employed from the step I3 to the step IB, and available in supply l I2. Thus, in such cases the-supplies Il and I2 will not be identical in proportion oi' water-to alcohol. vAccording to the present invention, the stills 22 and 21 are operated 'at the same pressure, and the alcohol-water mixture used as the solvent material pressure without change of composition..

Such a mixture `is known as a constant-boiling mixture -or as an azeotropic mixture, it being understood that these terms comprehend some specified total vapor pressure. The preferred pressure for the invention is atmospheric pressure, whether it be at sea level or at elevated altitudes. or whether it change slightly with weather conditions at any given location. Thus, the process conducted at atmospheric pressure at azeotropic mixture which is diiIerent from another azeotropic mixture employed in a process at atmospheric pressure in some mountainous location.

Numerous alcohols have been examined and found deilcient in one or more respects. Methyl alcohol does not form an azeotropic mixture at '160 mm. N-butyl alcohol and isobutyl alcohols are not misclble with water in all proportions at ordinary room temperatures, and the azeotropic mixtures with water at one atmosphere pressure,

separate into two' liquid phases at the chill temare room temperature or lower, be `subjected in the process.

peratures, which to which they may Secondary butyl alcohol as an azeotropic mixture with water at 32% by weight at 760 mm.

with water to provide a solvent mixpressure on cooling remains single phase until l 3 C. is attained, when ice crystals begin to form. It may be used in the present invention where the chilling process is carried out as recommended, in operating at 760 mm. pressure or thereabouts. This temperature will change with the pressure. Thus, the secondary butyl alcohol may be a border-line alcohol for the invention and may or may not be used according to the operating pressure'and the choice of the chill temperature. Among the various alcohols the azeotropic mixtures with water and the alcohols: ethyl, n-propyl, isopropyl, secondary' butyl and tertiary-butyl, have a suitably low capacity for dissolving naphthalene at the cold temperatures described, to permit high recovery of naphthalene in the process, without the necessity to recirculate large dissolved quantities of naphthalene in the undistilled recycled portion of the mother liquor. Another advantage of the azeotropic mixtures pf these alcohols with water at normal pressure of one atmosphere is their low `boiling points. These are all close to the melting point of naphthalene, and under the,

C. for water, whereby a low be utilized for heat in the boiling point of pressure steam may process. Likewise, the cold temperature required may readily be attained with 'tap water,

or by ordinary cooling by room air even to 30 C. in summer, or 20 C. or less in winter.-

'I'he following table shows various properties of azeotropic mixtures at 1 atmosphere pressure.

Mole percent Weight B P water C percent o water Alcolici Ethyl N-propyl. Isoproiy Secon ary-buty Tertiary-butyl In the case of aqueous n-propyl alcohol, the composition of the azeotropic mixture is substantially constant as the pressure changes from '100 to 800 mm.

In the case of aqueous isopropyl alcohol, the composition of the azeotropic mixture changes from about 30 to 33 mole percent water in a change from 700 to 800 min. pressure.

In the case of aqueous mixtures of ethyl alcohol, the composition oi the azeotropic mixture changes from about 4 to 10 mole percent of water in a change from 640 to 800 mm. pressure.

Thus, it will be appreciated that any considerable change of pressure for the stills will alter the composition of the solvent mixture, and

hence its solubility characteristics for the proc-- 0 ing description of a continuous operationof the apparatus shown in Fig. 2.

In Fig. 2 the process generally described in Fig. 1 is carried out in a practical manner. 'Ihe amounts given are for use of azeotropic mixture of water and isopropyl alcohol, and for operais from a fresh solvent reservoir 3d in which the liquid is designated MS (mixed solvent). The

liquid MS is substantially free from impurities ci the naphthalene derived in process, but it may contain some naphthalene, the presence of which could be considered an impurity in the solvent for some other use or process. The liquid MS is supplied to mixer 3l at the rate of 203 gallons per hour.

The liquid from supply 33 is a recycled liquor of the process, and contains solids toan appreciable extent, as well as solvent mixture. It is termed also RL (recycled liquor) and'is supplied at the rate of 1330 lbs. of solid and 1150 gallons of solvent mixture, per hour.

A coil 35 is supplied to heat or cool the solventgoing to mixer 3l so that as it combines with the melted naphthalene, the temperature oi the resulting solution is about 70 C., which is but a vaporized. The residue in the still is the impuritles 33 removed from the naphthalene Scand it maybe drawn from the still at the rate of 235 lbs. per hour., It corresponds to 4.7% impurities in the original naphthalene l0.

From the still 62 the solvent vapors are removed in line 30, cooled or ally condensed in heat exchanger 35, r zur denser 35,- and led to reservoir 3d Iat the rate of 203 gallons per hour. The heat-exchanger 55 is in series with the heating coil 6i andthe line 30. l

From the foregoing itwill be observed that 1353 gallons per'hour oi' solvent enter the mixer few degrees above the temperature at which crystallization begins.,

The hot solution is kthen run continuously into a crystallizer 35 wherein it is cooled. It may be operated as a two-stage cooler, jacket 3l! being cooled by returning liquor L in pipe line :i3-3S, andjacket d0 at the exit end being .cooled by tap water. fr rotary conveyer'i urges the resulting slush forward into a settling chamber d2. Crystals tend to settle, and liquor/L is drawn oi at the top into pipe 38. At the bottom of the lower end, a suitable dispersing means @i5 discharges mired solvent MS leaving reservoir 3d in pipe t6 at the rate of 427 gallons per hour, to wash the settling crystals. upwardly and combine With the original mother liquor discharged from the crystallizer 30.

The crystals settle into continuous conveyordevice 48 having an expressing action, as a result of which liquid is collected at d3 and carried away in the pipe 50 to distributor 5i Ain the settling chamber d2 above the distributor 05.

'Ihe conveyor d3 discharges a mass of wet crystals of naphthalene which are melted in a suitable device 53, and then introduced into a still 56 operated at a pressure which is atmospheric at approximately sea-level for the details of the procedure being described. In the still the mixed solvent originally adhering to the crystals is vaporized, and when condensed at 55 is led by pipe line 5l to solvent reservoir 3d. Approximately 427 gallons per hour are thus returned to the reservoir. It will be appreciated that some naphthalene vapor may also pass out of the still and contaminate the alcohol, but for recycling uses in'the process, this is not in fact any disadvantage.

At the bottom of the still melted naphthalene 58 is drawn off at the rate of 4765 lbs. per hour. This is the puried naphthalene" sought by the process.

The liquor L in line 38-33, heated when entering pipe 39 by.I passage through heat-excha i. r jacket 3l, is split at point 53 into major and minor portions. Major portion passes by pipe 33 as the described liquor RL. The minor portion passes in pipe 50, through a heating coil di', into still 52, where at atmospheric pressure it is The washings move ci which only 203 gallons per hour are substantially new, fresh or vered solvent. This is 15%. It follows. then, that at point 53, the liquor L in line 33 is divided in the proportion of l5 parts and 85 parts. f Y

The percentage of liquor recycled in the practice of the invention is a matter ofadjustment and control, according to many factors. The purity oi naphthalene supplied and required, the economics, the extent of dilution by washing are all factors aiecting the division of the liquor into portions to m recycled and distilled.' The proportion is not critical for the invention. Because of the fact that a portion is recycled, the invention requires an aaeotropic mixture. En more of liquor L that can be recycled, the lower the cost oi operation of the process.

If the apparatus should be changed'so that the liquor L is less diluted by the solvent entering it by way of pipes d5 and 50, the recycled proportion should be less than that described.

to secure a purication of the same order. Other apparatus may be used to separate crystals and liquor, and to wash the crystals, and such a changel may materially affect this division oi the liquor at point 59.

In the specic eple illustrated in Fig. 2, it

is to `be understood that use of a more crude'.

naphthalene than that described will require that a less proportion of liquor be recycled, to

secure the same degree of purity in the product,' while using also the same amounts of liquid for the washings of thecrystals.

In carrying out the invention the final purity aimed at is of course an arbitrary one. Practically, however, in addition to a high-solidica-Y tion point (S. PJ, a standard wash test purity of d to 5 is aimed at. 'Ihe test is sometimes referred to as the Barrett test. It is briefly as follows: Mix. 1-1 cc. of benzol with 7 cc. o f molten naphthalene to be tested. The solution is then added to 7 co. of concentrated (95%) sulfuric acid in a square one-ounce bottle. The mixture is shaken for 5 seconds and then allowed to stand 5 minutes. The color and clarity oi the acid layer are compared to a series of standards in similar bottles.

The invention is not limited to use of the particular steps and apparatus described herein by way of illustrating the invention. Numerous modifications and departures are contemplated as falling within the scope of the invention as expressed in the appended claims.

l. The method of purifying naphthalene which comprises forming a hot solution of naphthalene in a mixed solvent of water and an alcohol selected from the group consisting of ethyl, n-propyl, isopropyl, secondary-butyl and tertiarybutyl alcohols, said mixed solvent being the azeotropic mixture for the pressure hereinafter y condensed in conrecited, cooling the solution to effect formation of crystals of naphthalene in a resulting singlephase liquor in the form of a slush of liquor and crystals, separating crystals and liquor, washing the crystals with said mixed solvent substantially lacking in impurities for naphthalene, combining the washings and said liquor, distilling residual alcohol and water from the washed naphthalene at a chosen pressure at which said mixed solvent is azeotropic to recover the content of said mixed solvent for reuse in the process, whereby the residue of such distillation is puriiled naphthalene, dividing the said combined liquors into two portions, distilling said one portion at said chosen pressure to recover the content of said mixed solvent for reuse in the process, and mixing the other portion of liquor with a quantity of said mixed solvent which is substantially free from impurities from naphthalene to provide a-body of said mixed solvent, and forming a hot solution of naphtha.- lene to be puried in said body in repeating the process herein recited.

2. The process of claim 1 in which the chosen pressure is atmospheric pressure for the locale of the process.

3. The process of claim 1 in which the selected alcohol is isopropyl alcohol. i

4. The process of claim 1 in which the chosen pressure is atmospheric-pressure for the locale of the process, and in which the selected alcohol is isopropyl alcohol.

5. A continuous process for the purification of naphthalene permitting a substantially closed circuit for materials recycled in the process, which comprises (1) mixing acontinuous supply of naphthalene to be puried with a continuous mixed supply of mixed solvent to f orm a hot solution, said mixed solvent being essentially a mixture of water and an alcohol selected from the group of ethyl, n-propyl, iso-propyl, secondary-butyl and tertiary-butyl alcohols, said mixed solvent being azeotropic at the pressurehereinafter recited, said mixed supply including a portion of cornbined liquor as hereinafter recited from the supply RL hereinafter recited containing impurities derived in the process from the naphthalene subject ,to purification by the process and including a portion of said mixed solvent derived from a supply MS hereinafter recited which is substantially free from impurities from said naphthalene; (2) cooling said hot solution to effect formation of crystals of naphthalene in a resulting single-phase liquor in the form of a slush of liquor and crystals; (3)' withdrawing said crystals from association with'siiii liquor phase against a washing stream of said'mixed solvent derived from said supply MS; (4) combining the resulting washings with said liquor phase' to provide a supply L; (5) dividing said supply L into one portion as supply RL above recited and a remaining portion; (6) conveying said remaining portion to a continuous still; (7) distilling mixed solvent from said still at a chosen pressure at which said solvent is azeotropic; (8) condensing the vapors from said still and conducting the resulting liquid to said suply MS, the residue in said still being impurities derived from the naphthalene; (9) conveying the wet washed crystals of naphthalene to a continuous still in melted form; (l0) distilling mixed solvent from said still at said chosen pressure; (11) condensing the vapors from said still and conducting the resulting liquid to said supply MS; and (l2) withdrawing puriiled naphthalene from said last mentioned still.

6. The process of claim 5 in which the pressure for operating the stills is atmospheric.

'1. The process of claim 5 in which the selected alcohol is isopropyl alcohol.

8. The process of claim 5 in which the pressure for operating the stills is atmospheric and in which the selected alcohol is isopropyl.

9. Apparatus for continuously purifying soluble crystallizable material comprising essentially and in sequence, a mixer in which to form a relatively hot solution of said material in a solvent, a cooler for said solution in which to convert said solution into a solid phase of said ma- 'terial and a single liquor phase, a separator for said two phases, a still for removing residual solvent from the said solid phase, and in further association with said' elements, means to divide said liquor phase into two portions, a still in which to recover solvent from one portion, means to lead said other portion to the mixer, means to wash residual liquor from the solid phase with fresh solvent, means to combine the resulting washings with said liquor phase prior to dividing the same into said two portions, a container for a supply of said fresh solvent, means to lead the distillate of said two stills to said container, and means to lead fresh solvent from said container to said mixer for combining fresh solvent with said portion of liquor entering the mixer.

DONALD F. WILCOCK.

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